1. Field of the Invention
The present invention relates to a mold additive for continuous casting of steel.
2. Discussion on Related Art
Mold additives are used in continuous casting of steel for preventing oxidation of the surface of molten steel which contacts with the inner surface of the mold, heat retention, absorption of nonmetal inclusions and lubrication between the mold and the cast product.
A mold additive normally comprises metal oxides such as SiO.sub.2, CaO, Al.sub.2 O.sub.3, MgO and MnO as a flux base material and, if necessary, a melting property regulator comprising metal oxides such as Na.sub.2 O, K.sub.2 O, Li.sub.2 O, and B.sub.2 O.sub.3 and a metal fluoride such as CaF.sub.2, AlF.sub.3, NaF and LiF for regulation of melting point and viscosity and a carbonaceous powder for regulation of melting rate.
When a mold additive is added at the surface of the molten steel in a mold, the portion which contacts with the surface of the molten steel melts to form a molten slag layer and unmolten slag layer on the molten slag layer which cover the surface of the molten steel.
The molten slag layer generally used is mainly composed of 25-45 wt % of SiO.sub.2, 25-45 wt % of CaO, 1-20 wt % of Al.sub.2 O.sub.3, 5-20 wt % of Na.sub.2 O and 5-20 wt % of F, CaO/SiO.sub.2 being 0.5-1.8 and has physical properties of a viscosity of 0.5-15 poises at 1300.degree. C. and a melting point of 900.degree. C.-1250.degree. C.
When the performance of the mold additive is insufficient, there occur problems such as formation of pinholes due to oxidation of steel, breaking-out caused by sticking between the mold and the cast product and formation of cracks on the surface of cast product owing to nonuniform removal of heat.
Therefore, there have been made various proposals to prevent these defects. For example, Japanese Patent Kokai No. 60-234751 discloses a mold additive containing 3-35 wt % of a melting type oxide of 0.01-1 mm in particle size and Japanese Patent Kokai No. 57-41862 discloses a mold additive which is an additive for stationary casting comprising a flux base and, if necessary, a melting property regulator and additionally containing 0.5-15 wt % of zirconia.
However, in the case of the conventional mold additives, the amorphous slag formed upon melting contains some bubbles. These bubbles result in nonuniform cooling of the cast product at the surface of the mold in continuous casting of steel, which causes not only defects in the surface of the cast product, but also break-out. Thus, there have been problems in ensuring the quality of cast product and carrying out a stable operation. According to the above Japanese Patent Kokai No. 60-234751, one or more of magnesia, alumina, forsterite, zircon, wollastonite and silica which have been once molten in an electric furnace to enhance fire resistance and reduce reactivity is used as a slag forming base material to increase the melting temperature of the flux after use. Japanese Patent Kokai No. 57-41862 aims at preventing damage to the dipped nozzle by containing zirconia in the additive and this is not sufficient for decreasing bubbles in the molten slag.